In an LTE (Long Term Evolution) system, the CL-MIMO (closed-loop multiple-input multiple-output) technology is introduced. In the CL-MIMO, a terminal performs measurement on a channel state, and selects, according to a criterion, a PMI codebook most matching the channel from a given PMI (precoding matrix indication) codebook set and reports the PMI codebook to a base station. The base station selects a corresponding precoding matrix according to the PMI codebook reported by the terminal to perform weighting on downlink transmit data, so as to obtain an array gain of a transmit end. In the CL-MIMO, if different delay differences exist between channels of a transmit end RRU (Remote Radio Unit, remote radio unit), system performance deteriorates seriously. Therefore, to improve the system performance, channel correction of the RRU needs to be performed.
In the CL-MIMO, the transmit end may use a 2T2R (that is, two transmit channels and two receive channels) structure or a 4T4R (that is, four transmit channels and four receive channels) structure. In the existing LTE system, the 2T2R structure is mostly used. To improve the system performance and configure the system with the 4T4R structure, two feasible implementation manners exist: One implementation manner is that a new RRU with 4T4R is directly used to replace an existing 2T2R RRU; and the other implementation manner is that two existing 2T2R RRUs are combined into one RRU that has a 4T4R function. For the first implementation manner, direct replacement of the existing RRU causes an increase of costs and a waste of resources; and for the second implementation manner, because channels of different RRUs have relatively large delay differences, the system performance degrades. Therefore, joint channel correction needs to be performed on the two combined RRUs to keep the delay differences between channels of different RRUs consistent.
A method for joint channel correction between RRUs exists in the prior art and is applied to a TDD (time division duplex) system. As shown in FIG. 1, FIG. 1 is a schematic diagram of an existing method for correcting a channel of a combined RRU in a TDD system. In this method, an RRU0 and an RRU1 first complete self-correction, then the RRU0 and the RRU1 transmit a correction signal through a service transmit channel of one RRU of the RRU0 and the RRU1, a service receive channel of the RRU receives a correction signal looped back by an antenna, and joint channel correction of the RRU0 and the RRU1 is implemented by using the looped back correction signal.
In a process of implementing embodiments of the present disclosure, the inventor finds that at least the following defect exists in the prior art: The method for joint channel correction between RRUs provided in the prior art can be applicable only to the TDD system, and is not applicable to joint correction of RRUs in an FDD (frequency division duplex) system. This is because in the FDD system, a working frequency of a service transmit channel is not consistent with a working frequency of a service receive channel, a correction signal transmitted by the service transmit channel cannot be received by the service receive channel, and therefore, the foregoing method cannot be applied to channel correction between RRUs in the FDD system. In the prior art, a method for correcting a channel of a 2T2R RRU in an FDD mode does not exist. In this case, for a combined RRU, delay consistency among RRU channels cannot be ensured, thereby affecting the system performance.